A manufacturing method of a semiconductor wafer, such as a silicon single crystal wafer, generally includes a slicing step of slicing a single crystal ingot obtained by the Czochralski method (CZ method) into a thin disk wafers; a chamfering step of chamfering an outer circumferential portion to prevent breakage and chips of the wafer; a lapping step of flattening the wafer; an etching step of removing process strain residing in a surface layer of the wafer due to these steps; a mirror-polishing step of polishing a wafer surface into a mirror surface; and a cleaning step of removing contaminants such as a polishing agent and extraneous substances attached in the polishing step. A heat treatment or a grinding step other than the above steps may be added, the order of the steps may be changed, or the same step may be performed multiple times, as needed.
In recent years, a sallow trench isolation (STI) process or the other process is used as a minute device technique as semiconductor devices have been more integrated. Accordingly, formation of a circuit wiring pattern on a surface of a semiconductor wafer needs to form a flatter insulator film with a more uniform thickness on the semiconductor wafer; a method of flattening an insulator film by, for example, chemical mechanical polishing (CMP) is used in semiconductor device fabrication processes.
In the past, minute uneven shape (also referred to as waviness below) of a surface of a semiconductor wafer does not particularly affect the semiconductor device fabrication processes. On the contrary, STI causes the problem in that the thickness of an insulator film becomes nonuniform due to the above waviness, because CMP selectively polishes convex parts. Waviness is represented by a parameter called nanotopography. Nanotopography is an indicator representing flatness of a wafer surface and represents waviness of the wafer surface in a non-absorption state in a spatial wavelength ranging from 0.1 mm to several tens of millimeters.
Nanotopography is generally measured with an instrument such as Nanomapper made by ADE Corp., NanoPro made by KLA-Tencor Corp., or Dynasearch made by Raytex Corp. Since these instruments are optical instruments and take measurements by using surface reflection from an object to be measured, a subject wafer needs to have a mirror surface with a relatively high reflectance.
The values of nanotopography measured with these instruments for wafers that are subjected merely intermediate steps and hence have surfaces with low reflection, such as sliced wafers and ground wafers, are therefore inaccurate and unreliable.
In addition, factors that degrade nanotopography at the intermediate steps in the wafer manufacturing method in recent years include waviness that appears in a wire travelling direction when a single crystal ingot is sliced with a wire saw, and ring-shaped waviness that appears in a lapping step or a double-disc grinding step.
There is a proposed method of performing a double-disc grinding step, a double-side lapping step, and a double-side polishing step after slicing to reduce such waviness (See Patent Document 1, for example).
Patent Document 1 discloses that the double-disc grinding step removes a layer of strain and macro waviness components that are formed in a wafer surface during slicing and improves flatness of the wafer, and the subsequent double-side lapping step removes minute surface waviness created in the double-disc grinding step.
The size of the waviness created during slicing however may greatly vary depending on the performance of a slicing apparatus to be used, specifications of a wire, slicing conditions, and failure such as wire disconnection during slicing. Similarly, the size of the waviness appearing during the lapping step may vary depending on a turn table and a carrier. The size of the waviness appearing during the double-disc grinding may vary depending on relation of relative position between grinding wheels and a wafer, and quality of the grinding wheels.
Since it is difficult to remove residual waviness created on the wafer surface due to these factors in the mirror-polishing step, which is the final step, the size of waviness that appeared needs to be monitored and controlled at the intermediate steps, such as the slicing step or the lapping step, in the wafer manufacturing method. A conventional optical measurement method using surface reflection however has the problem in that the waviness cannot be precisely detected.
A conventional method proposed to measure the waviness of a semiconductor wafer having a low reflectance is to band-pass filtering warp data obtained with a capacitive shape measurement device by cutting off wavelength ranges of at least a short wavelength period of 1 mm or less and a long wavelength period of 50 mm or more (See Patent Document 2, for example).